U.S. patent application number 10/161035 was filed with the patent office on 2003-12-04 for dense refractory material for use at high temperatures.
Invention is credited to Duruz, Jean-Jacques, Nguyen, Thinh T., Nora, Vittorio de.
Application Number | 20030224220 10/161035 |
Document ID | / |
Family ID | 32097109 |
Filed Date | 2003-12-04 |
United States Patent
Application |
20030224220 |
Kind Code |
A1 |
Nguyen, Thinh T. ; et
al. |
December 4, 2003 |
Dense refractory material for use at high temperatures
Abstract
A component is made of or coated with a refractory material for
use at high temperature, e.g. in an aluminium production cell, an
arc furnace or an apparatus for treating molten metal. The
refractory material comprises particles of a refractory metal
compound selected from metal borides, silicides, nitrides, carbides
and phosphides, in an oxide matrix. The oxide matrix comprises a
bonding mixed oxide made of a single mixed oxide or a plurality of
miscible mixed oxides. The refractory material is obtainable from a
heat treated slurry that comprises: a colloidal and/or polymeric
oxide carrier, suspended particles of the refractory metal compound
and suspended metal oxide particles. The suspended refractory metal
compound particles and the suspended metal oxide particles are both
reactable with the colloidal and/or polymeric oxide to form the
bonding mixed oxide.
Inventors: |
Nguyen, Thinh T.; (Onex,
CH) ; Duruz, Jean-Jacques; (Geneva, CH) ;
Nora, Vittorio de; (Nassau, BS) |
Correspondence
Address: |
RANBAXY PHARMACEUTICALS INC.
SUITE 2100
600 COLLEGE ROAD EAST
PRINCETON
NJ
08540
US
|
Family ID: |
32097109 |
Appl. No.: |
10/161035 |
Filed: |
June 3, 2002 |
Current U.S.
Class: |
428/702 |
Current CPC
Class: |
F27D 27/00 20130101;
C04B 41/87 20130101; Y02P 10/25 20151101; F27D 1/0006 20130101;
C22B 9/05 20130101; C04B 41/507 20130101; C22B 21/064 20130101;
C04B 41/5037 20130101; Y02P 10/262 20151101; C04B 41/89 20130101;
F27D 1/1684 20130101; C22B 11/06 20130101; C25C 3/08 20130101 |
Class at
Publication: |
428/702 |
International
Class: |
B32B 018/00 |
Claims
1. A component which is made of or coated with a refractory
material for use at high temperature, the refractory material
comprising particles of a refractory metal compound in an oxide
matrix, the refractory metal compound being selected from metal
borides, silicides, nitrides, carbides and phosphides, the oxide
matrix comprising a bonding mixed oxide made of a single mixed
oxide or a plurality of miscible mixed oxides, the refractory
material being obtainable from a heat treated slurry that
comprises: a) a colloidal and/or polymeric carrier that comprises
colloidal and/or polymeric oxide of at least one metal; b)
suspended particles of the refractory metal compound that are
covered with an integral film of oxide of the metal of the
refractory metal compound, the oxide film being reactable upon heat
treatment with said colloidal and/or polymeric oxide to form a
mixed oxide comprised in said bonding mixed oxide; and c) suspended
metal oxide particles which are reactable upon heat treatment with
said colloidal and/or polymeric oxide to form a mixed oxide
comprised in said bonding mixed oxide, wherein the bonding mixed
oxide, including the mixed oxide formed from the reaction of the
oxide film and the colloidal and/or polymeric oxide, consists of: a
single mixed oxide when the metal of the suspended metal oxide
particles is the same as the metal of the suspended refractory
metal compound particles and the reactable oxide of said colloidal
and/or polymeric oxide is an oxide of one metal only; or a
plurality of miscible mixed oxides when at least one metal of the
suspended metal oxide particles is different to the metal of the
suspended refractory metal compound particles and/or when said
colloidal and/or polymeric oxide comprises reactable oxides of
different metals.
2. The component of claim 1, wherein the bonding mixed oxide
comprises a mixed oxide of the metal(s) of said colloidal and/or
polymeric oxide and at least one metal selected from titanium,
silicon, chromium, vanadium, zirconium, hafnium, niobium, tantalum,
molybdenum and cerium which is derived from the oxide film of said
refractory metal compound suspended particles and/or said suspended
metal oxide particles.
3. The component of claim 1, wherein said colloidal and/or
polymeric oxide is selected from colloidal and/or polymeric
alumina, ceria, lithia, magnesia, silica, thoria, yttria, zirconia,
tin oxide and zinc oxide, and mixtures thereof.
4. The component of claim 1, wherein the refractory metal compound
is titanium diboride and the bonding mixed oxide comprises
titanium-aluminium mixed oxide.
5. The component claim 1, wherein the bonding mixed oxide
constitutes at least 10 weight %, typically at least 30 weight %
and preferably at least 50 weight %, of the oxide matrix.
6. The component of claim 5, wherein the oxide matrix further
comprises non-reacted particles of said colloidal and/or polymeric
oxide.
7. The component of claim 5, wherein the oxide matrix further
comprises non-reacted particles of said suspended metal oxide.
8. The component of claim 5, wherein the bonding mixed oxide
consists of a single mixed oxide.
9. The component of claim 5, wherein the oxide matrix comprises a
plurality of mixed oxides that are at least partly miscible to form
the bonding mixed oxide.
10. The component of claim 1, comprising a body coated with the
refractory-material coating, the coating comprising at least two
different grades of refractory compounds in one or more layers.
11. The component of claim 10, wherein the coating comprises a
plurality of layers, each layer containing only one grade of
refractory metal compound.
12. The component of claim 1, wherein the refractory material is
producible from a colloidal and/or polymeric carrier that contains
different grades of colloidal and/or polymeric particles.
13. The component of claim 1, wherein the bonding mixed oxide and
the refractory metal compound are substantially inert to and
insoluble in molten aluminium.
14. The component of claim 1, which during use is in contact with
molten aluminium, molten fluoride-containing electrolyte and/or
oxidising gas.
15. The component of claim 14, wherein the oxide matrix further
comprises an aluminium-wetting agent consisting of a metal oxide
that is reactable with molten aluminium to form alumina and an
alloy of aluminium and the metal of the wetting agent making the
refractory material aluminium-wettable.
16. The component of claim 15, wherein the aluminium-wetting agent
is selected from oxides of manganese, iron, cobalt, nickel, copper,
zinc, molybdenum, the Lanthanides and rare earth metals.
17. The component of claim 15, comprising a carbon body coated with
the aluminium-wettable refractory material, the aluminium-wettable
refractory material being bonded to the carbon body through an
anchorage layer which is free from constituents that are miscible
or able to react with molten aluminium upon heat treatment, the
anchorage layer forming a barrier against molten aluminium.
18. The component of claim 17, wherein the composition of the
aluminium-wettable refractory material consists of the composition
of the anchorage layer plus the aluminium-wetting agent.
19. The component of claim 17, wherein the aluminium-wettable
refractory material is covered with a start-up layer applied from a
slurry made of particulate wetting oxide in a polymeric and/or
colloidal binder, the start-up layer constituting upon heat
treatment a temporary layer protecting said aluminium-wettable
refractory material and promoting wetting of the aluminium-wettable
refractory material by molten aluminium.
20. The component of claim 17, wherein the anchorage layer
comprises a heat treated colloidal and/or polymeric carrier which
contains only one grade of colloidal and/or polymeric
particles.
21. The component of claim 17, wherein the aluminium-wettable
refractory material is producible from a colloidal and/or polymeric
carrier that contains different grades of colloidal and/or
polymeric particles.
22. The component of claim 14, which is a component of an aluminium
electrowinning cell.
23. The component of claim 22, which is a cathode, part of a cell
bottom or a cell sidewall.
24. The component of claim 1, which is a holder for arc electrodes
or a carbon arc electrode having at least one inactive surface
coated with said refractory material.
25. The component of claim 1, which is a component of an apparatus
for treating molten metal.
26. A slurry which upon heat treatment produces a refractory
material for use at high temperature, the refractory material
comprising particles of a refractory metal compound in an oxide
matrix, the refractory metal compound being selected from metal
borides, silicides, nitrides, carbides and phosphides, the oxide
matrix comprising a bonding mixed oxide made of a single mixed
oxide or a plurality of miscible mixed oxides, the slurry
comprising: a) a colloidal and/or polymeric carrier that comprises
colloidal and/or polymeric oxide of at least one metal; b)
suspended particles of the refractory metal compound that are
covered with an integral film of oxide of the metal of the
refractory metal compound, the oxide film being reactable upon heat
treatment with said colloidal and/or polymeric oxide to form a
mixed oxide comprised in said bonding mixed oxide; and c) suspended
metal oxide particles which are reactable upon heat treatment with
said colloidal and/or polymeric oxide to form a mixed oxide
comprised in said bonding mixed oxide.
27. A method of manufacturing a component made of a refractory
material or coated with a refractory material, comprising providing
a slurry according to claim 26 and heat treating the slurry to
react the colloidal and/or polymeric oxide with the oxide film of
the refractory metal compound and with the suspended metal oxide
particles to form said bonding mixed oxide of the oxide matrix that
contains the refractory metal compound particles, wherein the
bonding mixed oxide consists of: a single mixed oxide when the
metal of the suspended metal oxide particles is same as the metal
of the suspended refractory metal compound particles and the
reactable oxide of said colloidal and/or polymeric oxide is an
oxide of one metal only; or a plurality of miscible mixed oxides
when at least one metal of the suspended metal oxide particles is
different to the metal of the suspended refractory metal compound
particles and/or when said colloidal and/or polymeric oxide
comprises reactable oxides of different metals.
28. The method of claim 27, for producing a component made of the
refractory material.
29. The method of claim 27, for producing a component coated with
the refractory material by applying one or more layers of the
slurry onto the component and heat treating the slurry.
30. The method of claim 29, wherein a plurality of layers are
applied from one or more slurries, each applied layer being allowed
to dry and/or subjected to a heat treatment before application of
the next layer.
31. An apparatus for operation at high temperature which comprises
at least one component as defined in claim 1 that is exposed during
operation to high temperature conditions.
32. The apparatus of claim 31, which is an aluminium electrowinning
cell, said component being a cathode, a cell sidewall or part of a
cell bottom.
33. The apparatus of claim 31, which is an arc furnace for treating
steel, said component being a coated carbon arc electrode or a
holder for arc electrodes.
34. The apparatus of claim 31, which is an apparatus for treating
molten metal, said component being exposable to the molten metal
and/or an oxidising media.
35. The apparatus of claim 34, wherein said component is exposable
to moving contact with molten metal.
36. The apparatus of claim 34, wherein the component is a vessel
for containing molten metal or a stirrer for stirring molten
metal.
37. A method of producing aluminium in a cell as defined in claim
32 which contains alumina dissolved in a fluoride-containing molten
electrolyte and in which said component is exposed to at least one
of molten electrolyte, cathodically reduced aluminium and
anodically evolved gas, the method comprising electrolysing the
molten electrolyte containing the dissolved alumina to cathodically
reduce aluminium and anodically evolve gas.
38. The method of claim 37, comprising producing aluminium on a
drained cathode.
39. A method of treating iron or steel in a furnace as defined in
claim 33 in which during operation said coated carbon arc electrode
or holder for arc electrodes is exposed to a high temperature
oxidising media, comprising passing an electric current through the
electrode to produce molten iron.
40. A method of treating a molten metal in an apparatus as defined
in claim 34, comprising imparting a relative movement between the
molten metal and said component.
41. A component which during use is exposed to molten aluminium,
comprising a body coated with an adherent multi-layer protective
coating which during operation is exposed to molten aluminium, the
protective coating having an outer layer which is wettable by
molten aluminium by penetration thereof into the outer layer, and
an aluminium-repellent layer underneath forming a barrier to molten
aluminium on the body which prevents exposure of the body to molten
aluminium.
42. The component of claim 41, wherein the body is made of carbon
or carbon-containing material.
43. The component of claim 42, wherein the aluminium-wettable outer
layer contains a wetting agent that draws molten aluminium into the
coating.
44. The component of claim 43, wherein the wetting agent is
selected from oxides of manganese, iron, cobalt, nickel, copper,
zinc, molybdenum, the Lanthanides and rare earth metals.
45. The component of claim 41, wherein the aluminium-repellent
layer is free of any wetting agent.
46. The component of claim 41, wherein at least one of the
aluminium-wettable outer layer and the aluminium-repellent layer
comprises a refractory material as defined in claim 1.
47. The component of claim 41, wherein the outer layer is covered
before use with a start-up layer applied from a slurry made of
particulate wetting oxide in a polymeric and/or colloidal binder.
Description
FIELD OF THE INVENTION
[0001] The invention relates to components which are made of or
coated with a refractory material for use at high temperature, in
particular cathode blocks of cells for the electrowinning of
aluminium from alumina dissolved in a cryolite-based molten
electrolyte, arc electrodes of steel arc furnaces and components of
molten metal purification apparatus.
[0002] The invention relates also to a slurry for producing this
refractory material, a method to manufacture these components,
apparatus using these components and methods of operating such
apparatus.
BACKGROUND OF THE INVENTION
[0003] Carbonaceous materials are important engineering materials
used in diverse applications such as aircraft bodies, electrodes,
heating elements, structural materials, rocket nozzles,
metallurgical crucibles, pump shafts, furnace fixtures, sintering
trays, induction furnace susceptors, continuous casting dies, ingot
moulds, extrusion canisters and dies, heat exchangers, anodes, high
temperature insulation (porous graphite), gas diffusers, aerospace
structural materials, bearings, substrates in electronics industry,
brazing and joining fixtures, diamond wheel moulds, nozzles, glass
moulds etc. Although carbonaceous materials have properties which
make them useful for the applications mentioned above, the
resistance to oxidation is one property which has limited the use
of these materials. Much effort is therefore underway to improve
the resistance to oxidation of such materials.
[0004] Traditional methods of protecting carbonaceous materials
have involved the deposition of adherent and highly continuous
layers of materials such as silicon carbide or metals such as
aluminium. The deposit of such materials has normally been carried
out by techniques such as vapour deposition (both PVD and CVD) or
by electrochemical methods. Vapour deposition is an extremely slow
and costly process and additionally may not be carried out for
large parts such as electrodes. It is also known to plasma spray
alumina/aluminium onto the sides of carbon anodes used as anodes
for aluminium electrowinning, but this coating method is expensive.
Other techniques such as electrochemical methods are limited in the
type of materials that may be applied as coatings, and size
limitations again may be present.
[0005] Various types of TiB.sub.2 or RHM layers applied to carbon
substrates have failed due to poor adherence and to differences in
thermal expansion coefficients between the titanium diboride
material and the carbon substrate.
[0006] Recently, protective coatings of refractory hard material
applied from a slurry have been successfully used on carbon
components, in particular cathodes of aluminium production cells.
Such slurries have been disclosed in U.S. Pat. No. 5,364,513
(Sekhar/de Nora).
[0007] U.S. Pat. No. 5,651,874 (de Nora/Sekhar) describes a
carbon-containing component of a cell for the production of
aluminium by the electrolysis of alumina dissolved in a
cryolite-based molten electrolyte, which cell component is
protected from attack by liquid and/or gaseous components of the
electrolyte or products produced during cell operation by a coating
of particulate refractory hard metal boride and a colloid bonding
applied from a slurry of the boride in a colloidal carrier which
comprises at least one of colloidal alumina, silica, yttria, ceria,
thoria, zirconia, magnesia, lithia, monoaluminium phosphate or
cerium acetate.
[0008] U.S. Pat. No. 5,728,466 (Sekhar/de Nora) discloses a carbon
cathode for the electrowinning of aluminium, to which a hard
surface is provided by adding to the surface of the carbon cathode
a layer containing particulate refractory hard metal boride and a
colloidal binder which when the carbon cathode is heated reacts
with the refractory hard metal boride and carbon from the cathode
or from a carbon-containing atmosphere.
OBJECTS OF THE INVENTION
[0009] An object of the invention is to provide a component made of
or coated with a refractory material from a slurry, which is
resistant to aggressive environments and can be used in aluminium
electrowinning cells, in steel arc furnaces and apparatus for
treating molten metal and other high temperature applications.
[0010] Another object of the invention is provide a slurry for
producing refractory coatings or bodies which are optionally
wettable by molten metal, electrically conductive and
electrochemically active for cathodic metal reduction reaction, in
particular for the production of aluminium form alumina dissolved
in a fluoride-containing molten electrolyte.
[0011] A major object of the invention is to provide slurry-applied
coatings of refractory hard material to protect components of
carbon or other materials, in particular carbon components of cells
for the electrowinning of aluminium or aluminium purification
apparatus, which coatings have a high density and an improved
mechanical resistance, in particular against delamination.
[0012] A preferred object of the invention is to provide a
component made of or coated with refractory material which can be
used in corrosive environments, such as oxidising media or gaseous
or liquid corrosive agents at elevated temperature, having an
improved resistance against oxidation, corrosion and erosion and
which has an improved electrical conductivity, electrochemically
active and/or physico-chemical properties.
[0013] A further object of the invention is to provide aluminium
electrowinning cell components with a slurry-applied coating
protecting against oxidation, corrosion and erosion, in particular
coated cathodes or drained cathodes with enhanced electrical
conductivity, electrochemical activity and physico-chemical
properties (aluminium-wettability).
[0014] Another object of the invention is to provide electrodes, in
particular for arc furnaces for the production of steel, coated on
their inactive surfaces with a coating protecting against premature
oxidation.
[0015] Yet another object of the invention is to provide components
for the treatment of molten metals protected with a slurry-applied
coating wettable by the molten metal and protecting against
oxidation and erosion of the component.
[0016] An object of the invention is to provide a component covered
with a coating protecting the component against wear and
controlling the component's lifetime, the coating's thickness
and/or composition being readily adjustable for different parts of
the component that may in use be exposed to different to wear
conditions.
SUMMARY OF THE INVENTION
[0017] It has been observed that particles of refractory metal
compounds, e.g. borides such as TiB.sub.2, suicides, nitrides,
carbides and phosphides, are surface oxidised when exposed to air
or another oxidising media which produces an oxide film of the
metal of the refractory metal compound. The present invention is
based on the insight that the existence of the oxide film on
particles of refractory metal compounds can be used to modify and
improve known coatings, e.g. those described in the abovementioned
U.S. Pat. No. 5,651,874.
[0018] The invention concerns the improvement of the cohesion of
the refractory metal compound particles within the refractory
material, the density of the refractory material and its mechanical
and chemical resistance. These improvements are achieved by
suspending a particulate metal oxide in the colloidal and/or
polymeric slurry in addition to the suspended particulate
refractory metal compound. Upon heating, the suspended particulate
metal oxide reacts with the polymeric and/or colloidal metal oxide
to form a mixed oxide that is at least partly miscible with the
mixed oxide produced from the reaction between the polymeric and/or
colloidal bonding metal oxide and the oxide surface of the
refractory metal compound particles.
[0019] Upon reaction, the refractory metal compound particles are
not merely bonded together by mixed oxide joints between them but
are bonded within a coherent mixed oxide matrix formed by the mixed
oxides of the colloidal and/or polymeric oxide reacted on the one
hand with the suspended particulate metal oxide and on the other
hand with the surface oxide of the refractory metal compound.
[0020] The high cohesion of the constituents of this refractory
material reduces the risk of cracks and increases the
impermeability of the refractory material to infiltration of
aggressive constituents from the environment during use.
[0021] One aspect of the invention relates to a component which is
made of or coated with a refractory material for use at high
temperature. The refractory material comprises particles of a
refractory metal compound in an oxide matrix. The refractory metal
compound is selected from metal borides, silicides, nitrides,
carbides and phosphides. The oxide matrix comprises a bonding mixed
oxide made of a single mixed oxide or a plurality of miscible mixed
oxides.
[0022] The refractory material is obtainable from a heat treated
slurry that comprises:
[0023] a) a colloidal and/or polymeric carrier that comprises
colloidal and/or polymeric oxide of at least one metal;
[0024] b) suspended particles of the refractory metal compound that
are covered with an integral film of oxide of the metal of the
refractory metal compound, the oxide film being reactable upon heat
treatment with said colloidal and/or polymeric oxide to form a
mixed oxide comprised in said bonding mixed oxide; and
[0025] c) suspended metal oxide particles which are reactable upon
heat treatment with said colloidal and/or polymeric oxide to form a
mixed oxide comprised in said bonding mixed oxide.
[0026] The bonding mixed oxide, including the mixed oxide formed
from the reaction of the oxide film and the colloidal and/or
polymeric oxide, is a single mixed oxide when the metal of the
suspended metal oxide particles is the same as the metal of the
suspended refractory metal compound particles and the reactable
oxide of the colloidal and/or polymeric oxide is an oxide of one
metal only, e.g. a slurry consisting of suspended particles of
surface oxidised titanium diboride and of titanium oxide in
colloidal alumina that produces a bonding mixed oxide consisting of
titanium-aluminium mixed oxide.
[0027] The bonding mixed oxide, including the mixed oxide formed
from the reaction of the oxide film and the colloidal and/or
polymeric oxide, consists of a plurality of miscible mixed oxides
when at least one metal of the suspended metal oxide particles is
different to the metal of the suspended refractory metal compound
particles and/or when the colloidal and/or polymeric oxide
comprises reactable oxides of different metals, e.g. a slurry
consisting of suspended particles of surface oxidised titanium
diboride and of magnesium oxide in colloidal alumina that produces
a bonding mixed oxide consisting of miscible titanium-aluminium
mixed oxide and magnesium-aluminium mixed oxide.
[0028] When the metals of the suspended metal oxide and the
refractory metal compound are different, it is advantageous to use
constituents of the slurry that produce mixed oxides having a great
miscibility. The greater the miscibility of the mixed oxides the
greater is the ratio of the bonding mixed oxide in the oxide matrix
which increases the matrix's stability and density.
[0029] The bonding mixed oxide may comprise a mixed oxide of the
metal(s) of said colloidal and/or polymeric oxide and at least one
metal selected from titanium, silicon, chromium, vanadium,
zirconium, hafnium, niobium, tantalum, molybdenum and cerium which
is derived from the oxide film of said refractory metal compound
suspended particles and/or said suspended metal oxide
particles.
[0030] The colloidal and/or polymeric oxide may be selected from
colloidal and/or polymeric alumina, ceria, lithia, magnesia,
silica, thoria, yttria, zirconia, tin oxide and zinc oxide, and
mixtures thereof.
[0031] For instance, the refractory material comprises titanium
diboride as the refractory metal compound and a titanium-aluminium
mixed oxide-containing bonding mixed oxide. This refractory
material can be obtained from a slurry of colloidal alumina
containing suspended particles of surface oxidised titanium
diboride and of titanium oxide or another metal oxide, e.g. silica
or magnesia, that forms a mixed oxide with alumina which is
miscible with titanium-aluminium mixed oxide.
[0032] Usually, the bonding mixed oxide constitutes at least 10
weight %, typically at least 30 weight % and preferably at least 50
weight %, of the oxide matrix. The oxide matrix may further
comprise non-reacted particles of the colloidal and/or polymeric
oxide and/or non-reacted particles of the suspended metal
oxide.
[0033] As stated above, the bonding mixed oxide may consist of a
single mixed oxide or a plurality of miscible mixed oxides. When
the bonding mixed oxide consists of miscible mixed oxides it can be
saturated with a miscible mixed oxide. Thus, in addition to the
bonding mixed oxide, the oxide matrix may comprises in a separate
phase the mixed oxide which is in excess to saturation. This may
happen when the mixed oxides forming the bonding mixed oxide are
only partly miscible.
[0034] The component may be a body that is coated with the
refractory-material coating, the coating comprising at least two
different grades of refractory compounds in one or several layers.
For example, the coating comprises a plurality of layers, each
layer containing only one grade of refractory metal compound.
[0035] To produce thick coatings or self-sustaining bodies, it is
preferred that the refractory material is produced from a colloidal
and/or polymeric carrier that contains different grades of
colloidal and/or polymeric particles, as taught in EP 0 932 589
(Sekhar/Duruz/Liu). Combinations of different grades of colloidal
and/or polymeric carrier improve the packing of the coating
particles and reduce the risk of cracks when the coating is dried
and/or heat treated.
[0036] When the component is used for applications in which it
comes into contact with molten aluminium or another metal, the
bonding mixed oxide and the refractory metal compound are
preferably substantially inert to and insoluble in the molten
metal.
[0037] The constituents of the slurry producing the refractory
material can be such that the refractory material is resistant to
attack by molten fluoride-containing electrolyte and/or oxidising
gas.
[0038] For certain applications in which the component contacts
molten metal, the oxide matrix further comprises a wetting agent
consisting of a metal oxide that is reactable with the molten metal
to form an oxide by transfer of the oxygen from the wetting agent
to the molten metal and an alloy of the molten metal and the metal
of the wetting agent.
[0039] For instance, when the refractory material comprises an
aluminium-wetting agent and is exposed to molten aluminium, the
molten aluminium reacts with the aluminium-wetting agent to form
alumina and an alloy of aluminium and the metal of the wetting
agent permitting infiltration of aluminium in the refractory
material without dissolution of the bonding oxide or the refractory
metal compound. The molten aluminium infiltration makes the
refractory material aluminium-wettable without dissolving it. The
aluminium-wetting agent is typically selected from oxides of
manganese, iron, cobalt, nickel, copper, zinc, molybdenum, the
Lanthanides and rare earth metals.
[0040] In one embodiment, the component comprises a carbon body
coated with the aluminium-wettable refractory material containing
the aluminium-wetting agent. The aluminium-wettable refractory
material is bonded to the carbon body through an anchorage layer
which is free from constituents that are miscible or able to react
with molten aluminium. The density of the anchorage layer and its
inertness to molten aluminium is such that the anchorage layer
forms a barrier to molten aluminium, i.e. no molten aluminium can
infiltrate the anchorage layer and penetrate into the carbon body
to cause damage, as demonstrated in Example 4.
[0041] Advantageously, the composisiton of the aluminium-wettable
refractory material is the same as the composition of the anchorage
layer plus the wetting agent. Because of the composition
compatibility, the anchorage layer and the aluminium-wettable
refractory material are intimately bonded upon heat treatment by a
continuous oxide matrix extending therethrough.
[0042] The aluminium-wettable refractory material may be covered
with a start-up layer applied from a slurry made of an
aluminium-wetting agent in a polymeric and/or colloidal binder. As
opposed to the aluminium-wettable refractory material, this
start-up layer constitutes upon heat treatment a temporary layer,
the temporary layer protecting the aluminium-wettable refractory
material, in particular against oxidising gas and/or molten
electrolyte attack during start-up in an aluminium electrowinning
cell. The start-up layer also promotes wetting of the
aluminium-wettable refractory material by molten aluminium. During
use and upon wetting, the start-up layer may be washed away.
[0043] As mentioned above, the thickness of layers applied from
colloidal and/or polymeric slurries can be increased by combining
different grades of colloids and/or polymers. For example, when the
anchorage layer is a current carrying layer and of a composition
that is free of constituents enhancing the conductivity, it is
preferable to limit its thickness by applying it from a slurry that
contains a single grade colloid or polymer. For aluminium
production cathodes, the thickness of the anchorage layer is
typically of the order of 100 to 150 micron or below.
[0044] Conversely, a layer of the aluminium-wettable refractory
material is rendered well conductive by being infiltrated during
use with molten aluminium that reacts with the aluminium-wetting
agent to form alumina and a well conductive alloy of aluminium and
the metal of the wetting agent. Therefore, the thickness of such a
refractory material layer can be increased to increase the layer's
lifetime without adverse effect on its electrical conductivity. For
this purpose, the aluminium-wettable refractory material may be
produced from a slurry that comprises a multi-grade colloidal
and/or polymeric carrier. For aluminium production cathodes, the
thickness of the layer of aluminium-wettable material is typically
of the order of 1 to 3 mm.
[0045] The overall electrical resistance of this composite coating,
i.e. anchorage layer plus aluminium-wettable refractory material,
is much lower, typically from about 100 to 1000 times lower, than
that of prior art slurry-applied coatings of equivalent thickness,
e.g. as disclosed in EP 0 932 589, which are made of materials
whose electrical resistivity is of the order of the resistivity of
the anchorage layer. At room temperature, the overall electrical
resistivity of the composite coating according to the invention is
typically of the order of 1 .OMEGA., whereas prior art coatings of
equivalent thickness have an overall resistivity of the order of
500 .OMEGA..
[0046] Usually, the useful lifetime of the start-up layer is short,
typically less than 24 hours after start-up. After a few hours the
aluminium-wettable refractory material is completely wetted by
molten aluminium rendering the start-up layer superfluous.
Therefore, the start-up layer's thickness can be quite small, e.g.
comparable to the thickness of the anchorage layer, and produced
from a slurry containing a single grade colloid or polymer.
[0047] Another aspect of the invention is a slurry which upon heat
treatment produces the refractory material described above.
[0048] A further aspect of the invention is a method of
manufacturing the above described component. The method comprises
providing a slurry as described above and heat treating the slurry
to react the colloidal and/or polymeric oxide with the oxide film
of the refractory metal compound and with the suspended metal oxide
particles to form the bonding mixed oxide of the oxide matrix that
contains the refractory metal compound particles.
[0049] As mentioned above, the entire component may be made of the
refractory material or only part thereof. In particular, the
component may be coated with the refractory material by applying
one or more layers of the slurry onto the component and heat
treating the slurry. Moreover, a plurality of layers may be applied
from one or more slurries, each applied layer being allowed to dry
and/or subjected to a heat treatment before application of the next
layer.
[0050] Yet another aspect of the invention is a component which
during use is exposed to molten aluminium. The component comprises
a body, typically made of carbon or carbon-containing material,
coated with an adherent multi-layer protective coating which during
operation is exposed to molten aluminium. The protective coating
has an outer layer which is wettable by molten aluminium by
penetration thereof into the outer layer, and an
aluminium-repellent layer underneath forming a barrier to molten
aluminium on the body which prevents exposure of the body to molten
aluminium.
[0051] Usually, the aluminium-wettable outer layer contains a
wetting agent, e.g. as disclosed above, that draws molten aluminium
into the coating. Conversely, the aluminium-repellent layer is
preferably free of any wetting agent. At least one of the
aluminium-wettable outer layer and the aluminium-repellent layer
may comprise a refractory material as described above.
[0052] To enhance wetting of the aluminium-wettable outer layer by
molten aluminium, the outer layer can be covered before use with a
start-up layer applied from a slurry made of particulate wetting
oxide in a polymeric and/or colloidal binder, as disclosed
above.
APPLICATIONS OF COMPONENTS OF THE INVENTION
[0053] The invention also relates to an apparatus for operation at
high temperature which comprises at least one component that is
made of or coated with the above described refractory material that
is exposed during operation of the apparatus to high temperature
conditions.
[0054] As stated above, the component of the invention is
particularly suited for use in an electrolytic cell for
cathodically producing aluminium from alumina dissolved in a
fluoride-containing molten electrolyte. The component of the
invention may be used as a cathode, in particular a drained
cathode, part of the cell bottom or a cell sidewall.
[0055] A further application of the above described component
relates to steel arc furnaces for treating steel to produce iron,
in particular as a holder for arc electrodes or as a carbon arc
electrode having at least one inactive surface coated with the
refractory material.
[0056] Furthermore, the component may be used in an apparatus for
treating molten metal, such as molten aluminium, magnesium, iron,
steel or copper. During use, the component is exposed to the molten
metal and/or an oxidising media.
[0057] This apparatus can be used for separating molten metal from
impurities and/or separating constituents of an alloy metal by
centrifugal and/or gravitational force.
[0058] The apparatus optionally comprises means for imparting a
rotary motion to the molten metal usually about a substantially
vertical axis and is so arranged that during use at least part of a
wear-exposed surface of the refractory material of the component is
temporarily or permanently in contact with molten metal.
[0059] The contacting molten metal can be static or in motion
relative to the component's wear-exposed surface. The component of
the invention may be a vessel containing the molten metal, a
stirrer for imparting a movement to the metal, a stator that in use
dips in the molten metal and is arranged to deliver treating fluid
into the molten metal, a rotatable stirrer arranged to dip in and
rotate the molten metal during operation, or another type of
disperser or a part thereof.
[0060] The component may consist of a coated carbon-based or
carbide-based material, in particular petroleum coke, metallurgical
coke, anthracite, graphite, amorphous carbon or mixtures thereof.
Alternatively, the coated part of the coated component consists of
metal-based material.
[0061] Whereas the coating of invention has been described with
particular reference to components of aluminium electrowinning
cells, arc electrodes and metal treatment systems, the invention is
useful inter-alia for protecting the various engineering components
made of carbon or other materials listed at the outset.
[0062] Such components may have a carbonaceous substrate, or a
substrate of a metal, alloy, intermetallic compound, ceramic or
refractory material, to which the coating is applied.
[0063] Further aspects and details of the invention will become
apparent in the Detailed Description and in the appended
Claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0064] Embodiments of the invention will now be described by way of
example with reference to the accompanying schematic drawings,
wherein:
[0065] FIG. 1 shows a schematic cross-sectional view of an
aluminium production cell with carbonaceous drained cathodes having
a coating in accordance with the invention;
[0066] FIG. 2 schematically shows an arc electrode furnace
incorporating coatings according to the invention;
[0067] FIG. 3 shows an apparatus for the purification of a molten
metal having a carbonaceous stirrer protected with a coating
according to the invention;
[0068] FIG. 3a is an enlarged schematic sectional view of part of
the stirrer shown in FIG. 3; and
[0069] FIG. 4 schematically shows a variation of the stirrer shown
in FIG. 3.
DETAILED DESCRIPTION
[0070] Aluminium Electrowinning Cell
[0071] FIG. 1 shows an aluminium electrowinning cell comprising a
series of carbonaceous anode blocks 5 having operative surfaces 6
suspended over a drained sloping flattened generally V-shaped
cathode surface 21 in a fluoride-containing molten electrolyte 42
containing dissolved alumina.
[0072] The drained cathode surface 21 is formed by the surface of
an aluminium-wettable coating 20A applied to the upper surfaces of
a series of juxtaposed carbon cathode blocks 15 extending in pairs
arranged end-to-end across the cell. The aluminium-wettable coating
20A is deposited from on or more colloidal and/or polymeric
slurries according the invention, for instance as embodied in
Examples 1, 1a, 2 or 2a but preferably as disclosed in Example
4.
[0073] The cathode blocks 15 comprise, embedded in recesses located
in their bottom surfaces, current supply bars 22 of steel or other
conductive material for connection to an external electric current
supply.
[0074] The drained cathode surface 21 is divided by a central
aluminium collection groove 26 located between pairs of cathode
blocks 15 arranged end-to-end across the cell. The aluminium
collection groove 26 is situated at the bottom of the drained
cathode surface 21 and is arranged to collect the product aluminium
draining from the cathode surface 21. The aluminium collection
groove 26 is coated with an aluminium-wettable coating 20B
according to the invention.
[0075] The anode blocks 5 too are coated with a refractory coating
20C on their inactive surfaces as shown in FIG. 1 or only on their
top surface and shoulders, i.e. only the top surfaces and the upper
part of the lateral surfaces. The anodes are not coated on the
operative anode surfaces 6 which are immersed as such in the molten
electrolyte 42. This coating 20C does not need to be particularly
wettable to molten aluminium and a coating applied form a slurry as
described in Example 1 or 1a would be sufficient. However, to
further improve the coating's protection against oxidation, it is
advantageous to add oxide-free or partly oxidised metal particles,
e.g. oxide(s) of iron, copper and/or nickel, to the slurry, e.g. as
disclosed in Examples 1 and 2a. Further to improve the coating's
resistance it can also be made aluminium-wettable and wetted with
aluminium before use.
[0076] The cell comprises carbonaceous sidewalls 16 exposed to
molten electrolyte and to the environment above the molten
electrolyte, but protected against the molten electrolyte 42 and
the environment above the molten electrolyte with a coating 20D
according to the invention. The coating 20D may be of the same
composition as the anode coating 20C.
[0077] The method of application of the coatings 20A,20B,20C,20D
comprises applying to the surface of the component a colloidal
and/or polymeric slurry as specified above, followed by drying. The
cathode coating 20A and the collection groove coating 20B can be
heat treated before or after installation in the aluminium
production cell, and when the coating contains oxidised metal
particles, e.g. oxides of nickel, iron or copper, the reaction of
these particles with molten aluminium can be done before or during
use. However, when appropriate, the anode coating 20C and the
sidewall coating 20D should be heat treated and reacted with molten
aluminium before use in the cell since these components do not
contact molten aluminium during use.
[0078] The method of coating the components 5,15,16 of the present
invention by application of the slurry involves painting (by brush
or roller), dipping, spraying, or pouring the slurry onto the
components 5,15,16 and allowing to dry before another layer is
added. The coating 20A,20B,20C,20D does not need to be entirely dry
before the application of the next layer. It is preferred to heat
at least the final coating 20A,20B,20C,20D with a suitable heat
source so as to completely dry it and improve densification.
Heating and drying take place preferably at about 80-200.degree.
C., usually for half an hour to several hours, and further heat
treatments are possible.
[0079] The surfaces of the carbon components 5,15,16 to be coated
with this slurry may be treated by sand blasting or pickled with
acids or fluxes such as cryolite or other combinations of fluorides
and chlorides prior to the application of the coating
20A,20B,20C,20D. Similarly the surfaces may be cleaned with an
organic solvent such as acetone to remove oily products and other
debris prior to coating. These treatments will enhance the bonding
of the coatings to the component.
[0080] Before or after application of the coating 20A,20B,20C,20D
and before use, the surfaces of the components 5,15,16 can be
painted, sprayed, dipped or infiltrated with reagents and
precursors, gels and/or colloids.
[0081] In operation of the cell illustrated in FIG. 1, alumina
dissolved in the molten electrolyte 42 at a temperature of
750.degree. to 960.degree. C. is electrolysed between the anodes 5
and the cathode blocks 15 to produce gas on the operative anodes
surfaces 6 and molten aluminium on the aluminium-wettable drained
cathode coating 20A.
[0082] The cathodically-produced molten aluminium flows down the
inclined drained cathode surface 21 into the aluminium collection
grooves 26 onto the aluminium-wettable coating 20B from where it
flows into an aluminium collection reservoir for subsequent
tapping.
[0083] FIG. 1 shows a specific aluminium electrowinning cell by way
of example. It is evident that many alternatives, modifications and
variations will be apparent to those skilled in the art. For
instance, the cell may have one or more aluminium collection
reservoirs across the cell, each intersecting the aluminium
collection groove to divide the drained cathode surface into four
quadrants as described in PCT/IB00/00476 (de Nora). The anodes may
be made of inert materials and have an electrochemically active
structure of grid-like design to permit electrolyte circulation, as
for example disclosed in PCT/IB99/01739 (de Nora/Duruz).
[0084] Arc Furnace
[0085] The arc furnace shown in FIG. 2 comprises three consumable
electrodes 15A arranged in a triangular relationship. For clarity,
the distance between the electrodes 15A as shown in FIG. 2 has been
proportionally increased with respect to the furnace. Typically,
the electrodes 15A have a diameter between 200 and 500 mm and can
be spaced by a distance corresponding to about their diameter.
[0086] The electrodes 15A are connected to an electrical power
supply (not shown) and suspended from an electrode positioning
system above the cell which is arranged to adjust their height.
[0087] The consumable electrodes 15A are made of a carbon substrate
laterally coated with a coating 20 protecting the carbon substrate
from oxidising gas. According to the invention, the coating is
deposited from a colloidal and/or polymeric slurry applied onto the
carbon substrate as one or more layers which are dried and/or
cured. The coating may contain metal oxide particles which are
reactable with molten aluminium, e.g. as disclosed in Examples 2
and 2a, in which case the coating should be exposed before use to
molten aluminium so that the reactable particles react with
aluminium.
[0088] The method of application of the coating 20 is similar to
the method described in relation to the cathode blocks 15 described
above.
[0089] The bottom of electrodes 15A which is consumed during
operation and constitutes the electrodes' operative surface is
uncoated. The coating 20 protects only the electrodes' lateral
faces against premature oxidation.
[0090] The electrodes 15A dip in an iron source 41, usually
containing iron oxide or oxidised iron, such as scrap iron, scrap
steel and pig iron. Preferably, the iron source 41 further
comprises reductants selected from gaseous hydrogen, gaseous carbon
monoxide or solid carbon bearing reductants. The reductants may
also comprise non-iron minerals known as gangue which include
silica, alumina, magnesia and lime.
[0091] The iron source 41 floats on a pool of liquid iron or steel
40 resulting from the recycling of the iron source 41.
[0092] During use, a three phase AC current is passed through
electrodes 15A, which directly reduces iron from the iron source
41. The reduced iron is then collected in the iron or steel pool
40. The gangue contained in the reduced iron is separated from the
iron by melting and flotation forming a slag (not shown) which is
removed, for example through one or more apertures (not shown)
located on sidewalls of the arc furnace at the level of the
slag.
[0093] The pool of iron or steel 40 is periodically or continuously
tapped for instance through an aperture (not shown) located in the
bottom of the arc furnace.
[0094] Molten Metal Purification Aparatus
[0095] The molten metal purification apparatus partly shown in FIG.
3 comprises a vessel 45 containing molten metal 40', such as molten
aluminium, to be purified. A rotatable stirrer 10 made of
carbon-based material, such as graphite, is partly immersed in the
molten metal 40' and is arranged to rotate therein.
[0096] The stirrer 10 comprises a shaft 11 whose upper part is
engaged with a rotary drive and support structure 30 which holds
and rotates the stirrer 10. The lower part of shaft 11 is
carbon-based and dips in the molten metal 40' contained in vessel
45. At the lower end of the shaft 11 is a rotor 13 provided with
flanges or other protuberances for stirring the molten metal
40'.
[0097] Inside the shaft 11, along its length, is an axial duct 12,
as shown in FIG. 3a, which is connected at the stirrer's upper end
through a flexible tube 35 to a gas supply (not shown), for
instance a gas reservoir provided with a gas gate leading to the
flexible tube 35.
[0098] The axial duct 12 is arranged to supply a fluid to the rotor
13. The rotor 13 comprises a plurality of apertures connected to
the internal duct 12 for injecting the gas into the molten metal
40', as shown by arrows 51.
[0099] The lower part of the shaft 11, i.e. the immersed part and
the interface region at or about the meltline 14 of the shaft, as
well as the rotor 13 are coated with a coating 20E deposited from a
colloidal and/or polymeric slurry according to the invention which
improves the resistance to erosion, oxidation and/or corrosion of
the stirrer during operation.
[0100] As shown in FIG. 3, the upper part of shaft 11 is also
protected with a coating 20F, against oxidation and/or corrosion.
The upper part of the carbon-based shaft 11 is coated with a thin
coating of refractory material 20F providing protection against
oxidation and corrosion, whereas the coating 20E protecting the
immersed part of the shaft 11 and the rotor 13 is a thicker coating
of refractory material providing protection against erosion,
oxidation and corrosion. Such a coating gradation is appropriate
for batch processes during which the stirrer is alternatively
exposed to molten metal and the atmosphere. For continuous
purification processes, the coating may be of uniform
thickness.
[0101] Likewise, surfaces of the vessel 45 which come into contact
with the molten metal may be protected with an aluminium-wettable
coating containing oxidised metal particles reactable with molten
aluminium according to the invention, as described in relation to
the anodes, cell sidewalls or arc furnace electrodes. The coating
is thereby further protected against oxidation by a surface film of
aluminium.
[0102] The method of application of the coating 20E,20F is similar
to the method described in relation to the cathode blocks 15
described above.
[0103] During operation of the apparatus shown in FIG. 3, a
reactive or non-reactive fluid, in particular a gas 50 alone or a
flux, such as a halide, nitrogen and/or argon, is injected into the
molten metal 40' contained in the vessel 45 through the flexible
tube 35 and stirrer 10 which dips in the molten metal 40'.
[0104] The stirrer 10 is rotated at a speed of about 100 to 500 RPM
so that the injected gas 50 is dispersed throughout the molten
metal in finely divided gas bubbles. The dispersed gas bubbles 50,
with or without reaction, remove impurities present in the molten
metal 40' towards its surface, from where the impurities may be
separated thus purifying the molten metal.
[0105] The stirrer 10 schematically shown in FIG. 4 dips in a
molten metal bath 40' and comprises a shaft 11 and a rotor 13. The
stirrer 10 may be of any type, for example similar to the stirrer
shown in FIG. 3 or of conventional design as known from the prior
art. The rotor 13 of stirrer 10 may be a high-shear rotor or a pump
action rotor.
[0106] In FIG. 4, instead of coating the entire shaft 11 and rotor
13, parts of the stirrer 10 liable to erosion are selectively
coated with a coating according to the invention.
[0107] The interface portion at and about the meltline 14 of the
carbon-based lower part of the shaft 11 is coated with a refractory
interface coating 20E.sub.1, for instance over a length of up to
half that of the shaft 11. Excellent results have been obtained
with a coating over a third of shaft 11. However, the length of
coating 20E.sub.1 could be a quarter of the length of shaft 11 or
even less, depending on the design of stirrer 10 and the operating
conditions.
[0108] In addition to the interface portion of such stirrers, other
areas may be liable to erode, again depending on the design and
operating conditions of the stirrers. The schematically shown
stirrer 10 in FIG. 4 illustrates further coated surfaces which are
particularly exposed to erosion. The lower end of the shaft 11
adjacent to the rotor 13 is protected with a coating 20E.sub.2, the
lateral surface of rotor 13 is protected with a coating 20E.sub.3
and the bottom surface of the rotor 13 is coated with a coating
20E.sub.4.
[0109] For each specific stirrer design, the coating or different
coatings on different parts of the stirrer, such as coatings
20E.sub.1, 20E.sub.2, 20E.sub.3 and 20E.sub.4 shown in FIG. 4, may
be adapted as a function of the expected lifetime of the stirrer.
For optimal use, the amount and location of such coatings can be so
balanced that they each have approximately the same lifetime.
[0110] In an alternative embodiment (not shown), the coating on
such stirrers may be continuous as illustrated in FIG. 3 but with a
graded thickness or composition so as to adapt the resistance
against erosion to the intensity of wear of each part of the
stirrer, thereby combining the advantages of the different coatings
shown in FIG. 4.
[0111] Various modifications can be made to the apparatus shown in
FIGS. 3, 3a and 4. For instance, the shaft shown in FIG. 3 may be
modified so as to consist of an assembly whose non-immersed part is
made of a material other than carbon-based, such as a metal and/or
a ceramic, which is resistant to oxidation and corrosion and which,
therefore, does not need any coating, whereas the immersed part of
the shaft is made of carbon-based material protected with a coating
according to the invention. Such a composite shaft would preferably
be designed to permit disassembly of the immersed and non-immersed
parts so the immersed part can be replaced when worn.
[0112] Likewise, a carbon-based non-immersed part of the shaft may
be protected from oxidation and corrosion with a coating and/or
impregnation of a phosphate of aluminium, in particular applied in
the form of a compound selected from monoaluminium phosphate,
aluminium phosphate, aluminium polyphosphate, aluminium
metaphosphate, and mixtures thereof. Suitable coating and/or
impregnation compositions are disclosed in U.S. Pat. No. 5,534,119
(Sekhar). It is also possible to protect the non-immersed part of
the shaft with a coating and/or impregnation of a boron compound,
such as a compound selected from boron oxide, boric acid and
tetraboric acid. Suitable coating and/or impregnation compositions
are disclosed in U.S. Pat. No. 5,486,278 (Manganiello/Duruz/Bell)
and in co-pending application WO97/26626 (de
Nora/Duruz/Berclaz).
[0113] In a modification, the coating of the invention may simply
be applied to any part of the stirrer in contact with the molten
metal, to be protected against erosion, oxidation and/or corrosion
during operation.
[0114] The invention will be further described in the following
examples.
EXAMPLE 1
[0115] A slurry according to the invention was prepared by
suspending a refractory hard metal compound consisting of 47.5 g
surface-oxidised particulate spherical TiB.sub.2 (-325 mesh) having
a TiO.sub.2 surface film and a metal oxide in the form of 2.5 g
TiO.sub.2 (-325 mesh) in a colloidal carrier consisting of 20 ml
colloidal Al.sub.2O.sub.3 (NYACOL.RTM. Al-20, a milky liquid with a
colloidal particle size of about 40 to 60 nanometer) and 1 ml PEG
(polyethylene glycol) which increases the viscosity of the slurry
and enhances its capacity to be applied by painting as well as the
adherence and coherence of the final coating.
[0116] This slurry produces upon heat treatment an oxide matrix of
titanium-aluminium mixed oxide from the reaction of the colloidal
oxide Al.sub.2O.sub.3 and TiO.sub.2 present as suspended oxide
particles and oxide film covering the suspended TiB.sub.2
particles. The oxide matrix contains and bonds TiB.sub.2
particles.
EXAMPLE 1a
[0117] The constituents of the slurry of Example 1 may be changed
as shown in the following Table in which each line represents
possible combinations of constituents:
1 Suspended Surface- Oxidised Colloidal or Suspended Metal
Refractory Metal Polymeric Oxides Oxides Compounds Al.sub.2O.sub.3
TiO.sub.2, MgO or SiO.sub.2 TiB.sub.2, SiC, TiC or TiN TiO.sub.2
Al.sub.2O.sub.3 or MgO SiC or SiN SiO.sub.2 Al.sub.2O.sub.3 or MgO
TiB.sub.2, TiC or TiN
EXAMPLE 2
[0118] Another slurry according to the invention was prepared by
suspending a refractory hard metal compound consisting of 92.5 g
particulate needle-shaped surface-oxidised TiB.sub.2 (-325 mesh)
having a TiO.sub.2 surface oxide film, an aluminium-wetting agent
in the form of 2.5 g particulate Fe.sub.2O.sub.3 (-325 mesh) and a
metal oxide in the form of 2.5 g TiO.sub.2 (-325 mesh) in a colloid
consisting of a combination of two grades of colloidal
Al.sub.2O.sub.3, namely 28 ml of a first grade of colloidal
Al.sub.2O.sub.3 (NYACOL.RTM. Al-20, a milky liquid with a colloidal
particle size of about 40 to 60 nanometer) and 24 ml of a second
grade of colloidal Al.sub.2O.sub.3 (CONDEA.RTM. 10/2 Sol, a clear,
opalescent liquid with a colloidal particle size of about 10 to 30
nanometer).
[0119] This slurry produces upon heat treatment a matrix of mixed
oxides containing titanium-aluminium mixed oxide and a small amount
of iron-titanium-aluminium mixed oxide from the reaction of
TiO.sub.2, Fe.sub.2O.sub.3 and Al.sub.2O.sub.3. This matrix
contains and bonds the TiB.sub.2 and Fe.sub.2O.sub.3 particles.
EXAMPLE 2a
[0120] Example 2's slurry composition consists of an
aluminium-wetting agent (Fe.sub.2O.sub.3) and a reaction mixture
made of the colloid (Al.sub.2O.sub.3), the suspended refractory
metal compound (TiB.sub.2) the suspended metal oxide (TiO.sub.2).
This Example can be modified by completely or partly substituting
the aluminium-wetting agent with copper oxide and/or nickel oxide,
and/or by varying the composition of the reaction mixture as in
Example 1a.
EXAMPLE 3
[0121] A further slurry for producing a temporary
aluminium-wettable start-up layer that can be used in combination
with coatings according to the invention, e.g. as disclosed in
Example 4, was prepared as follows. An amount of 60 g of surface
oxidised copper particles (-325 mesh) was suspended in a carrier
consisting of 13 ml of colloidal Al.sub.2O.sub.3 (7 ml NYACOL.RTM.
Al-20, a milky liquid with a colloidal particle size of about 40 to
60 nanometer and 6 ml CONDEA.RTM. 10/2 Sol, a clear, opalescent
liquid with a colloidal particle size of about 10 to 30 nanometer)
and 1 ml of PEG (polyethylene glycol) which increases the viscosity
of the slurry and enhances its capacity to be applied by painting
as well as the adherence and coherence of the final coating.
[0122] Upon heat treatment the slurry produces an alumina matrix
containing and bonding the oxidised copper particles.
[0123] As a modification, oxidised particles of nickel and/or iron
may be used to substitute in part or completely the oxidised copper
particles in colloidal alumina (CONDEA 25/5 with a pH>7).
EXAMPLE 4
[0124] Three carbon cathodes for use in a drained cell for the
production of aluminium were each coated with the slurries of
Examples 1, 2 and 3 as follows:
[0125] First, an anchorage layer having a thickness of about 100
micron was painted onto the exposed surface of the carbon cathode
from the slurry of Example 1. The anchorage layer was allowed to
dry for 30 minutes.
[0126] The anchorage layer was covered with an aluminium-wettable
layer obtained by painting 8 layers of the slurry of Example 2.
Each applied layer was allowed to dry for 30 minutes before
application of the next layer. The final aluminium-wettable layer
had a thickness of about 1.8 mm.
[0127] The aluminium-wettable layer was then covered with a
temporary start-up layer obtained by painting 1 layer of the slurry
of Example 3. The start-up layer had a thickness of about 100 to
150 micron.
[0128] The coating formed by the anchorage layer, the
aluminium-wettable layer and the start-up layer on the carbon
cathode was allowed to dry for 24 hours.
[0129] Two of the three cathodes were then covered with an
aluminium sheet having a thickness of about 1.5 cm and heated in an
oven at a temperature of about 850-900.degree. C. in air.
[0130] The first cathode was extracted from the oven after 30
minutes and allowed to cool down to ambient temperature.
Examination of a cross-section of the coating showed that aluminium
had infiltrated the start-up layer so that the coating was
superficially wetted by molten aluminium. No reaction between
aluminium and iron oxide had yet taken place.
[0131] The second cathode was extracted from the oven after 24
hours and allowed to cool down to ambient temperature. Examination
of a cross-section of the coating showed that aluminium had
infiltrated the start-up layer and the aluminium-wettable layer.
Part of the. aluminium had reacted with the Fe.sub.2O.sub.3 wetting
agent to form Al.sub.2O.sub.3 and Fe metal. Aluminium infiltration
had been stopped on the anchorage layer for lack of
aluminium-wetting agent which demonstrated that the anchorage layer
is an effective barrier layer against penetration of aluminium into
the carbon cathode.
[0132] The aluminium metal infiltration into the start-up layer and
the aluminium-wettable layer enhanced the conductivity of the
coating. At ambient temperature, the perpendicular electrical
resistance through the coating was less than 1 ohm after
infiltration versus more than 500 ohm before infiltration.
[0133] The coatings on both cathodes showed a continuous matrix of
titanium-aluminium mixed oxides between the anchorage layer and the
aluminium-wettable layer which guarantees an excellent adherence
between the two layers. In both cases the particles of TiB.sub.2
had not been oxidised by the heat treatment and wettability of the
coating by aluminium was very good. The angle of wettability was
less than 10 deg.
[0134] The third coated carbon cathode was used in an aluminium
production drained cell as follows:
[0135] The cathode covered with the dried coating according to the
invention was covered in the cell with a 1.5 cm thick sheet of
aluminium. The cell was heated to a temperature of about
850-900.degree. C. by passing an electrical current between the
cathode and facing anodes through carbon powder. Other start-up
heating procedures could also have been used, e.g. using gas
burners to generate heat.
[0136] After 30 minutes at 850-900.degree. C., the start-up coating
was superficially wetted by molten aluminium which constitutes a
barrier against damaging fluoride-based molten electrolyte
constituents, such as sodium compounds, and a cryolite based
electrolyte was filled into the cell.
[0137] The cell was further heated to 960.degree. C. at which
temperature the cell was operated under an electrolysis current
density of 0.8 A/cm.sup.2 to produce aluminium under conventional
steady state conditions.
[0138] As a modification, the cathode can be coated only with a
single layer of the composition of the anchorage layer if it is to
be used in a cell operating with a pool of aluminium. In this case,
high wettablility of aluminium is not critical and the coating may
not necessarily comprise an aluminium-wettable layer and a start-up
layer on the anchorage layer.
[0139] However, even for operation with an aluminium pool, the
coating preferably comprises an aluminium-wettable layer on the
anchorage layer for improved protection. Furthermore, to achieve
maximum protection of the carbon cathode, the coating comprises in
addition a start-up top layer.
* * * * *